1. Field of the Invention
The present invention relates to a method for producing a rare-earth magnet including a sintering process step and to a case for use in the sintering process.
2. Description of the Related Art
A rare-earth magnet is produced by pulverizing a magnetic alloy into powder, pressing or compacting the alloy powder in a magnetic field and then subjecting the pressed compact to a sintering process and an aging treatment. Two types of rare-earth magnets, namely, samarium-cobalt magnets and neodymium-iron-boron magnets, have found a broad variety of applications today. In this specification, a rare-earth magnet of the latter type will be referred to as an xe2x80x9cRxe2x80x94Txe2x80x94(M)xe2x80x94B type magnetxe2x80x9d, where R is a rare-earth element including Y, T is Fe or a mixture of Fe and Co, M is an additive element and B is boron. The Rxe2x80x94Txe2x80x94(M)xe2x80x94B type magnet is often applied to many kinds of electronic devices, because the maximum energy product thereof is higher than any other kind of magnet and yet the cost thereof is relatively low. However, a rare-earth element such as neodymium is oxidized very easily, and therefore great care should be taken to minimize oxidation during the production process thereof.
In the prior art process, a green compact (or as-pressed compact) obtained by compacting Rxe2x80x94Txe2x80x94(M)xe2x80x94B type magnetic alloy powder is sintered within a furnace after the compact has been packed into a hermetically sealable container (sintering pack 100) such as that shown in FIG. 1. This is because the sintered compact would absorb too much impurity existing inside the furnace and be deformed if the compact was laid bare inside the furnace. The sintering pack 100 includes a body 101 of the size 250 mmxc3x97300 mmxc3x9750 mm, for example, and a cover 102. Inside the pack 100, multiple green compacts 80 are stacked one upon the other on a sintering plate that has been raised to a predetermined height by spacers (not shown). The sintering pack 100 may be made of SUS304, for example, which is strongly resistant to elevated temperatures.
As shown in FIG. 2, multiple sintering packs 100 are stacked on a rack (or tray) 201 with spacers 202 interposed therebetween. Then, the rack 201 is loaded into a sintering furnace in its entirety and subjected to a sintering process. After the sintering process is finished, the cover 102 is removed from each of these sintering packs 100 and the sintered compact is unloaded from the pack 100 and then transferred to another container for use in an aging treatment.
According to the conventional process, while the sintering pack 100, in which the green compacts 80 are packed, is being transported to the rack 201, the green compacts 80 might fall apart due to vibration or might have their edges chipped, thus adversely decreasing the production yield. A green compact for an Rxe2x80x94Fexe2x80x94B type magnet, in particular, has usually been compacted with lower pressure compared to a ferrite magnet so that the particle orientation thereof in a magnetic field is improved. Thus, the strength of the green compact is extremely low, and great care should be taken in handling the compact.
Also, since the sintering pack 100 is provided with the cover 102, the green compacts 80 should be loaded and unloaded into/from the pack 100 manually. This is because it is difficult to load or unload them automatically. Thus, according to the conventional technique, productivity is hard to improve.
Moreover, although SUS304, the material for the sintering pack 100, is capable of withstanding an elevated temperature of 1000xc2x0 C. or more, the mechanical strength of the material at that high temperature is not so high. Due to the effect of elevated temperature on the mechanical strength of the material, if the pack 100 is continuously used in the heat for a long time, then the cover 102 might be deformed thermally or a chemical reaction might be caused between Ni contained in SUS304 and Nd contained in the green compacts 80 to erode the container. That is to say, the material is not sufficiently durable. Additionally, its lack of dimensional precision means that SUS304 is inadequate to use with automated processes.
Another problem with the use of SUS304 for sintering cases is that its thermal conductivity is relatively low. To obtain a sufficiently high heat conduction through the walls of sintering pack made of SUS304, the walls of the pack must be of a thin construction, which undesirably decreases their strength. Increasing the thickness of the walls of the pack to increase their strength results in poor conduction of heat, which increases the amount of required time required for the sintering process.
Furthermore, the present inventors have found that the sintered bodies are sometimes severely oxidized and deformed during the sintering process, even if the green compacts 80 are packed in the sintering pack 100.
An object of the present invention is providing a highly durable sintering case which exhibits excellent thermal conductivity and resistance to thermal deformation, and which will not react with rare earth elements.
Another object of the present invention is providing a sintering case, which is easily transportable and effectively applicable to an automated sintering furnace system and yet excels in shock resistance, mechanical strength and heat dissipation and absorption.
Still another object of the present invention is providing a method for producing a rare-earth magnet by performing sintering and associated processes using the inventive sintering case.
Still another object of the present invention is providing a method for producing a rare-earth magnet with high productivity by preventing compacts of rare-earth alloy powder from being oxidized during the sintering process.
A case according to the present invention is used in a sintering process to produce a rare-earth magnet. The case includes: a body with an opening; a door for opening or closing the opening of the body; and supporting means for horizontally sliding a sintering plate, on which green compacts of rare-earth magnetic alloy powder are placed. The supporting means is secured inside the body. At least the body and the door are made of molybdenum.
In one embodiment of the present invention, the body consists of: a bottom plate; a pair of side plates connected to the bottom plate; and a top plate connected to the pair of side plates so as to face the bottom plate. The door is slidable vertically to the bottom plate by being guided along a pair of guide members. The guide members are provided at one end of the side plates. In this particular embodiment, the upper end of the door is preferably folded to come into contact with the upper surface of the top plate when the door is closed.
In another embodiment of the present invention, the case may further include a plurality of reinforcing members that are attached to the body to increase the strength of the body. Each said reinforcing member includes: a first part in contact with the body; and a second part protruding outward from the first part. In this particular embodiment, the reinforcing members are preferably made of molybdenum.
In still another embodiment, the supporting means preferably includes multiple rods that are supported by the pair of side plates, and each said rod is preferably made of molybdenum.
Another case according to the present invention is used in a sintering process to produce a rare-earth magnet and is made of molybdenum.
Still another case according to the present invention is used in a sintering process to produce a rare-earth magnet and is made of molybdenum containing at least one of: 0.01 to 2.0 percent by weight of La or an oxide thereof; and 0.01 to 1.0 percent by weight of Ce or an oxide thereof.
Yet another case according to the present invention is used in a sintering process to produce a rare-earth magnet and contains 0.1 percent by weight or less of carbon and at least one of: 0.01 to 1.0 percent by weight of Ti; 0.01 to 0.15 percent by weight of Zr; and 0.01 to 0.15 percent by weight of Hf. The balance of the case is made of molybdenum.
Yet another case according to the present invention is used in a sintering process to produce a rare-earth magnet. The case includes: a casing including platelike members; and means for supporting a sintering plate, on which green compacts of rare-earth magnetic alloy powder are placed. The supporting means is provided inside the casing. The case further includes a reinforcing member provided on an outer surface of the casing.
In one embodiment of the present invention, the platelike members are preferably made of a material mainly composed of molybdenum.
An inventive method for producing a rare-earth magnet includes the steps of: pressing rare-earth magnetic alloy powder into a green compact; and sintering the green compact to form a sintered body using the case of the present invention.
In one embodiment of the present invention, the method may further include the steps of: placing the green compact on the sintering plate; loading the sintering plate, on which the green compact has been placed, into the case through the opening of the case; and closing the opening of the case with the door.
In this particular embodiment, the method may further include the steps of: performing a burn-off process on the green compact inside the case before the step of sintering the green compact is carried out; and conducting an aging treatment on the sintered body inside the case after the step of sintering the green compact has been carried out.
More specifically, the method further includes the steps of: placing the case on transport means; getting the case moved by the transport means to a position where the burn-off process is performed; and getting the case moved by the transport means to a position where the sintering step is performed.
Specifically, the opening of the case is opened before the aging treatment is performed.
In another embodiment of the present invention, powder of a neodymium-iron-boron permanent magnet may be used as the rare-earth magnetic alloy powder.
In still another embodiment, a molybdenum plate may be used as the sintering plate.
More particularly, one end of the molybdenum plate is preferably bent.
In still another embodiment, a getter (also called a xe2x80x9cgas absorbentxe2x80x9d) may be placed inside the case. In this particular embodiment, rare-earth magnetic alloy powder or a fragment of a green compact made of rare-earth magnetic alloy powder is preferably used as the getter.
A method for producing a rare-earth magnet of the present invention includes the steps of: (a) compacting alloy powder for the rare-earth sintered magnet to form a green compact; (b) loading the green compact into a case having a structure restricting a path through which gas flows between the outside and inside of the case, and placing a getter at least near the path; and (c) sintering the green compact by heating the case including the green compact inside in a decompressed atmosphere.
The getter may be placed inside of the sintering case. Alternatively, the getter may be placed outside of the sintering case.
Preferably, the getter includes rare-earth alloy powder, and the rare-earth alloy powder has substantially the same composition as the alloy powder for the rare-earth sintered magnet.
The average particle size of the rare-earth alloy powder is preferably smaller than the average particle size of the alloy powder for the rare-earth sintered magnet. In other words, the specific surface area of the rare-earth alloy powder is preferably greater than the specific surface area of the alloy powder for the rare-earth sintered magnet.
More preferably, the rare-earth alloy powder is magnetized.